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FAQ's for Wireless Video Transmission (Digital)
Q1: How many cameras can the system handle?
The number of cameras that can be handled on the RDT digital system, depends on several factors. There is a certain amount of capacity (bandwidth) available and each camera and each transmitter use up part of this bandwidth. Also, a camera at a greater distance from the receiver appears to use more bandwidth than a camera at a short distance. (see answer 3 below)
As a guide, a single transmitter can handle up to 7 cameras. This means 7 cameras, each with its own VIM (Video Input Module) can be hard wired to the transmitter via a router, the data being transmitted to a single receiver.
If the cameras are at separate locations, then up to 4 cameras, each with its own transmitter, can be handled. Note that in these configurations, for best results, the transmitters should be within radio coverage of each other. To allow all 4 transmitters to share the same channel, they use a LBT (Listen Before Transmit) strategy. Each transmitter listens to check if the channel is clear before transmitting. For this to work effectively, each transmitter must be able to hear the others, otherwise there may be interference. An additional feature, RTS / CTS also helps share the band and reduce collisions.
The 2.4GHz system allows for 3 non-overlapping channels and the 5.6GHz system, 6 non-overlapping channels. This means that a 2.4GHz system can be configured with up to 3 receivers each handling 4 transmitters giving a total of 12 cameras. A 5.6GHz system with 6 channels can therefore handle 24 cameras.
Q2: What Distances can be covered?
This question is related to the available bandwidth, as detailed in question 3 below. If there is only one camera being transmitted over a single TX / RX link, then all the system capacity (bandwidth) is available for this link. If there are a number of transmitters, then the bandwidth is shared between them, reducing the maximum distance possible for each camera.
The actual distances achieved depend on many factors, including: antenna type; clear line of sight* (see question14); antenna height; type of terrain etc.
We often see distances of 20 and 30km quoted but although these are theoretically possible in perfect conditions, we think a more realistic figure, with a high gain antenna is about 10km for 2.4GHz and 5km for 5.6GHz.
Q3: What is the Available Bandwidth?
Bandwidth is a confusing term when applied to an 802.11 network. It is probably more accurate and easier to understand if we refer to data rate. The RDT VideoWave Digital system uses uses 802.11a and g technology, which has a maximum data rate (or bandwidth) of 54Mbps. Much of this however is used for networking overhead and management, leaving about 14Mbps for actual data transmission. Each transmitter uses up an additional 2Mbps of overhead and each camera uses 1.5Mbps. (see question 13)
As a transmitter moves farther from the receiver, the signal strength and link quality gradually drops. The receiver adjusts for this by reducing the data rate to improve sensitivity. The distance to the transmitter can be increased until the data rate reaches its minimum useable speed. Once this point is reached, the total bandwidth (or system capacity) has been used up on this one link, so no other cameras can be supported.
Another way to view this is as follows: Assume a system has 4 transmitters sending to one receiver. If each transmitter sends a block of data every second and each block is 250mS long (it takes 250mS to transmit the block), then all 4 transmissions can be sent each second. If a transmitter is moved farther away and the receiver slows the data rate to 50%, then each block will be 500mS long and the link will only handle 2 blocks per second. If the speed drops to 25%, one block will take 1 second to transmit, using up the whole link capacity.
Q4: How many Channels?
The 802.11g network operates in the license free band, which is 2400MHz to 2483MHz, a total frequency bandwidth of 83MHz. The transmission is broadband, each channel being about 22MHz wide. In reality therefore, only 3 non overlapping channels can be fitted into the 83MHz. Typically, channels 1, 7 and 13 can be used, leaving a small space between each.
If for example, channel 7 is selected, it will overlap channels 5,6,8 and 9. It may also slightly affect channels 4 and 10. Other users on nearby channels may therefore affect performance, depending on their signal strength and how often they transmit.
The 802.11a is different. The total bandwidth available is from 5180MHz to 5825MHz but this is split into 3 sub-bands, A, B and C.
Band ‘A’ is from 5180MHz to 5320MHz and is reserved for low power indoor use.
Band ‘B’ from 5500MHz to 5700MHz is license free providing 11 channels, 6 non-overlapping. This is the band we normally use.
Band ‘C’ is from 5745MHz to 5825MHz providing a further 5 channels at higher power, but a license is required.
Q5: Can Repeaters increase distance?
No, normally a repeater will not increase distance significantly, but they are useful for getting around obstructions. If you require a point to point link but there is a large building in the way, a repeater can be used to relay the signal around the building.
One key point however, is that repeaters seriously reduce the available bandwidth. A repeater receives data for 50% of the time and transmits for the other 50%. This effectively halves the available bandwidth (data rate). There is normally 14Mbps available bandwidth (see question 3), but this will be reduced to 7Mbps if a repeater is used. This will only allow for a single camera link.
Because of this reduced bandwidth, a repeater will not significantly increase the maximum achievable distance (see question 1,2 and 3). It should also be remembered that a repeater normally requires an omni-directional antenna, which will further reduce the distance.
If a longer link is required, instead of a repeater, it is better to use a Receiver and Transmitter wired back-to-back, which effectively doubles the distance possible, without reducing the bandwidth.
Note that repeaters are not available at 5.6GHz but a repeater site can be created with a back-to-back receiver / transmitter as above.
Q6: Can the system handle telemetry?
With the RDT VideoWave Digital system, telemetry for PTZ cameras is built-in. The data is sent together with the video, so if there is a good video signal, telemetry data will also be received.
The Video Output Module (VOM) has an RS485 interface that can be connected to the controller (multi-drop for a number of cameras if required). The corresponding Video Input Module (VIM) has an RS485 interface to the camera.
The telemetry is compatible with most of the standard CCTV systems. The default settings are 9600, N, 8, 1 (9600 baud, No parity, 8 data bits, 1 stop bit). Other settings are available (factory programmed) as follows: 2400, 4800, 9600, 19,200, 38,400: Even, Odd or No parity. Check the label on the side of the VIM and VOM and make sure the camera and controller are set correctly.
Note that some PTZ controllers send data continuously (not just when the joystick is moved or a key is pressed). These are not suitable for operation over wireless as they use up too much bandwidth.
Q7: Why is there a delay on PTZ control?
The analogue signal from the camera is converted to digital in the VIM and compressed using MPEG-4 or H.264. This processing takes a short amount of time. At the receiving end, the VOM decompresses the data and converts back to a 1 V p-p composite video output. Again this processing takes time. There is therefore a small end to end delay.
Q8: What type of camera can I connect to the Videowave Digital system?
Any standard analogue video camera with a 1V p-p composite output may be used. The Digital name refers to the transmission method, not the camera connection. This enables advantages such as increased system capacity and range, while retaining the ease of connection found with conventional analogue cameras and recording equipment.
We can also supply just the wireless links without the encoder / decoder for use with IP cameras. In this case, most of the information in this document is still true, numbers of cameras, distances etc.
Q9: Are the transmitters, receivers, antennas, etc, all weatherproof?
The transmitter, receiver, VIM (Video input module), VOM (Video Output Module), router, repeater, and antennas are all suitable for external use. However, external antenna and video input connections must be sealed with self amalgamating tape.
Q10: What are the maximum cable distances between system components?
100m using CAT5 cable for data connections. e.g. from a VIM to a Transmitter, or from a Receiver to a VOM. Note that this distance can be doubled by adding a Router. e.g. VIM to Router (100mtrs) Router to Transmitter (100mtrs).
200m using standard 75Ω coax cable (URM70, RG59) e,g. Camera to VIM
1.2km for the RS485 connection using suitable twisted pair cable.
Q11: I need to transmit some alarm contacts, can this be done?
Yes, each VIM (Video input module) and VOM (Video output module) has two volt free contact inputs and two relay outputs, enabling contact signals to be sent both ways across the wireless link for operation with PIR detectors and switching of other external equipment (e.g. lighting). When using multiple PIR detectors to trigger preset positions on a dome camera, these will typically be wired directly to the dome (or local alarm card) to activate presets, then a global alarm output transmitted to activate an alarm input on the recording equipment.
The inputs are opto-isolated volt free contacts.
The relay outputs are rated at 5amps at 30V dc or 250V ac.
Q12: Are 20mA current loop and co-axial telemetry formats compatible?
No. When using 20mA current loop or co-axial telemetry, appropriate converters should be used to interface to the RS485 telemetry ports on the Videowave Digital VIM and VOM (e.g. BBV Rx100 dome interface, Baxall DAX converters). Please contact your telemetry equipment supplier to confirm your requirements.
Q13: Can the bandwidth setting for each camera be changed?
Yes it can but it requires a laptop and some networking knowledge.
Normally, the VIM (Video Input Module) is supplied with the camera bandwidth set to 1.5Mbps providing high quality images for most applications. For specialist applications requiring high bandwidth settings (e.g. recognition of moving vehicles) the VIM can be factory set to 3Mbps, if specified when ordering. This however may adversely affect system capacity. (see previous FAQs regarding system capacity and bandwidth). Lower bandwidth settings can also be factory set, offering an increase in system capacity at a reduced image quality for general observation. It is possible to have different VIMs on the same system set to different bandwidths (e.g Camera 1/VIM1 @ 2Mbps and Camera 2/VIM2 @ 1Mbps).
Another factor that affects the bandwidth and image quality, is the frame rate. This can be set between 25fps (pseudo real-time) and 1fps depending on the requirement. For general security applications, about 6fps is usually sufficient. At this setting, moving subjects will have a slight ‘jerky’ appearance. Again this can be discussed and specified at the time of ordering.
Q14: What is meant by “clear line-of-sight”?
When we refer to a clear line-of-sight for radio signals, this is different to a visual line-of-sight. You may be able to see the transmitting antenna from the receiver site, but this does not necessarily mean a good radio path.
A radio signal transmitted between two antennas will spread out forming an elliptical shape that is widest at the mid distance between the two antennas. The area within this ellipse is known as the Fresnel Zone and any obstructions within this zone will cause an obstruction and interference to the signal path.
Note that this Fresnel Zone is three dimensional and has both height and width. The maximum radius of the beam, at the centre, increases with the distance between the two antennas. As a guide, at 2.4GHz the radius for a particular signal path is:
1Km signal path 4 - 5Mtr radius
2Km signal path 5 - 6Mtr radius
5Km signal path 8 - 9Mtr radius
This means for example, that if the link is 2Km long, the antennas must be at least 5 to 6 metres above the ground and above any building or obstruction. There must also be the same 5 to 6 metres clearance to either side.
It should also be remembered that radio signals at 2.4GHz will not normally pass through buildings and they will be severely attenuated by vegetation, such as trees.
Even with a clear line-of-sight (as defined above) the ground, or nearby buildings, can cause reflections. These reflected signals will also arrive at the receiver and can interfere with the direct signal causing a reduction in signal strength or lost data.
